(1) FIELD OF THE INVENTION
The invention relates to the general field of integrated circuits, more particularly to the improved use of spin on glass.
(2) DESCRIPTION OF THE PRIOR ART
It is common practice to apply, to the surface of an integrated circuit, a layer of a spin on glass (SOG). As the name implies, a spin on glass is applied to the surface of the integrated circuit in liquid form, most commonly by spin coating. After drying, this coating is heated in an oxidizing atmosphere as a result of which it is converted into a glassy layer. Since it was originally applied as a liquid, this layer will have a planar upper surface; that is, the integrated circuit will be planarized. Additional steps in the manufacture of the integrated circuit, such as etching, can now proceed.
One problem associated with this method of planarization is that, during the application of the SOG, a thickened region, or edge bead, tends to form near the periphery of the wafer. This is a consequence of the fact that the SOG is applied in droplet form at the axis of the spinning wafer and then spreads out because of centrifugal force. A "SOG hump" is induced as a result of the top and edge rinse (in isopropyl alcohol or similar solvent) that is applied during the SOG coating process.
The above-mentioned SOG edge bead is undesirable for several reasons: During subsequent processing steps, such as plasma etching where the wafer is held in place by a set of tight fitting claws, the edge bead tends to act as a source of dust. The edge bead introduces stress into the SOG layer which, in turn, leads to the propagation of defects into the semiconductor layers immediately beneath it. Last, but not least, the presence of the bead defeats one of the main purposes of SOG application, namely planarization.
For all of these reasons it is common practice to remove the edge bead as part of the total process. A straightforward, but relatively expensive, way of achieving this is to provide a photoresist mask that protects the center of the wafer and to then remove the edge bead by etching in the normal way, following which the resist mask is removed and processing proceeds as normal. In order to avoid this extra masking and etching step a number of novel schemes for the direct removal of edge beads have been described in the prior art. For example, Koze et al. (U.S. Pat. No. 5,425,846 June 1995) stack multiple wafers, each of which has a similar edge bead, one on top of another and then subject the stack to a plasma etch. Etching proceeds from the wafers' edges inward and is terminated once the etch front has passed the edge beads.
Cuthbert et al. (U.S. Pat. No. 4,510,176 April 1985) direct a jet of SOG solvent at the wafer's periphery while the wafer is spinning. Care is taken to ensure that debris resulting from the SOG removal does not contaminate the wafer.
The approach of Brewer (U.S. Pat. No. 4,732,785 March 1988) is similar to that of Cuthbert et al. but is modified in that multiple treatments (of solvent) are given to the edge bead while at the same time the speed of rotation is gradually reduced.
Tanigawa et al. (U.S. Pat. No. 5,328,871 July 1994) remove the edge bead after the SOG has been applied to the wafer but before it has been processed into a glass. Under these conditions it is easier to remove. This is done using a jet of solvent directed at the spinning wafer at a point just inside the wafer's periphery.